reliability centered maintenance (rcm) for asset

of 202 /202

Author: hathuan

Post on 31-Dec-2016




5 download

Embed Size (px)














    OCTOBER 2010

  • ii


    I hereby declare that I carried out the work reported in this thesis in

    the Department of Electrical and Information Engineering, School of

    Engineering and Technology, College of Science and Technology,

    Covenant University, Ota, Nigeria under the supervision of Prof.

    C.O.A. Awosope and Prof. J.C. Ekeh.

    I also solemnly declare that no part of this report has been submitted

    here or elsewhere in a previous application for award of a degree. All

    sources of knowledge used have been duly acknowledged.



  • iii


    This is to certify that this thesis is an original research work undertaken by

    Anthony Uwakhonye ADOGHE (CU05GP0125) and approved by:

    1. Name: Prof. C.O.A. Awosope


    Signature: Date: 15th

    October, 2010

    2. Name: Prof. J.C. Ekeh


    Signature: Date: 15th

    October, 2010

    3. Name: Prof. J.C. Ekeh

    Head of Department

    Signature: . Date: 15th

    October, 2010

    4. Name: Dr. T.O. Akinbulire

    External Examiner

    Signature: .. Date: 15th

    October, 2010

  • iv


    This thesis is dedicated to God Almighty for his faithfulness and love towards

    me and to the service of Humanity.

  • v


    I am grateful to almighty God, the Author and finisher of my faith, for granting

    me access to his ceaseless revelation, wisdom and favour that saw me through

    my doctoral studies.

    My sincere appreciation goes to the Chancellor, Dr. David Oyedepo for the

    vision and mission of the University.

    Also, special thanks to the Vice Chancelor, the Registrar, the Deans of Colleges,

    the Heads of Department for their commitment and drive for excellence and

    sound academic scholarship.

    I also heartily appreciate and sincerely thank my supervisor, Prof. C.O.A.

    Awosope, whose encouragement, guidance and support enabled the successful

    completion of this thesis.

    I owe my deepest gratitude to my co-supervisor who is also my Head of

    Department Prof. J.C. Ekeh, for his good counsel, ever-ready willingness to

    assist and motivate, and more importantly his critical review of the work and

    useful suggestion in ensuring the success and speedy completion of this work.

    It is my pleasure to thank Prof. James Katende, the Dean of the College of

    Science and Technology (CST), for his support and encouragement all through

    the course of this work. Special thanks to the Dean of post Graduate School,

    Professor C.O. Awonuga and all my teachers at the postgraduate school. I thank

    all my friends and senior colleagues in the Department of Electrical and

    Information Engineering for their support and willingness to assist at very short

    notice during the course of this work. Engr. Gab I. Ezeokoli the Abule-Egba

    Business unit manager, Power Holding Company of Nigeria Plc is also highly

    appreciated for given me open access to their maintenance data used for this

    thesis. I also sincerely appreciate Dr.S.A. Daramola whose thesis provided useful

    guide for my writing.

    Lastly, but deliberately Iwant to specialy appreciate my wife and my children for

    their understanding, support and contributions to the success of this endeavour,

    God keep you all for me.

  • vi


    Title page i

    Declaration . ii

    Certification iii

    Dedication iv

    Acknowledgements v

    Table of Contents vi

    Lists of Symbols and Abbreviations x

    List of Figures and Diagrams ........ xii

    List of Tables xv

    Abstract xvi

    Chapter one: Introduction

    1.1 Background information.....................................................................1

    1.2 Research problem definition (statement of the problem)...................4

    1.3 Aim and objectives of the study........................................................ 8

    1.4 Research methodology...................................................................... 8

    1.5 Significance of the study...................................................................9

    1.6 Motivation of the study.....................................................................9

    1.7 Expected contribution to knowledge ................................................11

    1.8 Scope and limitation...........................................................................11

    1.9 Thesis organization............................................................................12

  • vii

    Chapter two: Literature Review

    2.1 Introduction..................................................................................... 14

    2.2 Maintenance approaches............................................................... 16

    2.3 The emergence of RCM............................................................... 17

    2.4 Evolution of maintenance............................................................. 18

    2.5 What is reliability? ................................................................. 26

    2.6 Reliability centred maintenance.............................................. 28

    2.7 Reliability engineering............................................................. 34

    2.8 Reliability engineering process................................................ 35

    2.9 Reliability evolution................................................................ 36

    2.10 Limitation of RCM.................................................................. 39

    2.11 Observations and Findings from the Literature Survey ..... 42

    2.12 The proposal of reliability centred maintenance (RCM) for asset

    management in electric power distribution system............................. 42

    2.13 Summary ..................................................................................... 45

    Chapter three: Theory of Reliability Evaluation

    3.1 Introduction.................................................................................... 46

    3.2 Definition and terminology .......................................................... 46

    3.3 Applied reliability indices............................................................ 52

    3.4 Maintenance Strategies.................................................................. 55

    3.5 Choosing an appropriate distribution model ................................. 55

    3.6 Modelling of life distribution function.......................................... 57

  • viii

    3.7 Exponentially distributed random variable................................... 59

    3.8 Weibull-distributed random variable......................................... 59

    3.9 Failure rate modelling for the RCM studies................................. 60

    3.10 Method of reliability evaluation ............ 60

    3.11 Ways for constructing the developed model.......................... 65

    Chapter four: Application of RCM model to PHCN network

    4.1 Introduction................................................................................. 80

    4.2 The network topology description............................................... 81

    4.3 Data collection and processing.................................................... 87

    4.4 Modelling of failure and repair processes.................................... 100

    Chapter five: Transformer Inspection and Maintenance:

    Probabilistic Models

    5.1 Introduction.................................................................................... 104

    5.2 Causes of transformer Failure............................................. 105

    5.3 Transformer maintenance model....................................... 108

    5.4 Equivalent mathematical models for transformer maintenance..112

    5.5 Sensitivity analysis of inspection rate on mean time to

    first failure (MTTFF). 115

    5.6 Analysis of the mean time to first failure .. 120

  • ix

    Chapter six: Estimating the remaining life of the identified

    distribution transformer

    6.1 Introduction................................................................................... 122

    6.2 Assets life cycle...................................................................... 123

    6.3 Techniques for asset management of transformers............ 124

    6.4 Performing maintenance plans............................................... 129

    6.5 Determination of the transition parameters for predicting the remaining life

    of an asset (distribution transformer)........................................... 131

    6.6 Determination of the steady - state probabilities.................... 133

    6.7 Determination of the mean time to first failure...................... 135

    6.8 Sensitivity analysis of failure rate on estimated remaining life of distribution

    transformer................................................................................... 139

    6.9 Discussion and analysis of results......................................... 146

    Chapter seven: Conclusion and recommendations

    7.1 Summary ....................................................................... 148

    7.2 Achievements ........... 149

    7.3 Recommendation .......................................................... 150

    Annexes 152

    References .. 179

  • x


    AENS Average energy not supplied per customer served

    AI Artificial Intelligent

    AM Asset Management

    ANN Artificial Neural Network

    ASAI Average Service Availability Index

    CAIDI Customer Average Interruption Duration Index.

    CAIFI Customer Average Interruption Frquency Index

    CA Condition Assessment

    CBM Condition Based Maintenance

    CiGre International council on large electric systems

    CM Corrective Maintenance

    cm Condition Monitoring

    CH4 Methane

    C2H2 Acetylene

    C2H4 Ethylene

    C2H6 Ethane

    CO Carbon monoxide

    CO2 Carbon dioxide

    DGA Dissolved gas analysis

    DP Degree of polymerization

    EPRI Electric Power Research Institute

    FA Fura Analysis

    FRA Frequency Response Analysis

  • xi

    HPP Homogeneous Poisson Process

    HST Hot Spot temperature

    HV High voltage

    H2 Hydrogen

    int. Interruption of voltage

    LOE Average loss of energy

    LTA Logic decision tree analysis

    LV Low Voltage

    MATLAB Matrix Laboratory

    MC Monte Carlo

    MM Maintenance Management

    MTBF Mean Time Between Failures

    MTTFF Mean Time To First Failure

    MTTR Mean Time To Repair

    MV Medium Voltage

    NHPP Non Homogeneous Poisson process.

    PHCN Power Holding Company of Nigeria

    PD Partial Discharge

    PM Preventive Maintenance

    RCM Reliability-Centered Maintenance

    RMS Root mean Square value

    SAIDI System Average Interruption Duration Index.

    SAIFI System Average Interruption Frequency Index

    UMIST: University of Manchester Institute of Science and Technology.

  • xii


    Figure 1.1: Project scope definition 12

    Figure 2.1: Overview of maintenance approches 16

    Figure 2.2: Reactive maintenance model 21

    Figure 2.3: Proactive maintenance model 22

    Figure 2.4: Composition of availability

    and its controlling parameters .. 35

    Figure 2.5: Logic of relating component maintenance

    System reliability with operating costs. 44

    Figure 3.1: Definitions of failures 50

    Figure 3.2: Total time for repair/replacement 51

    Figure 3.3: Outage time sequence 52

    Figure 3.4: Discrete-parameter Markov Model for the determination of the

    remaining life . 67

    Figure 3.5: Continuous Parameter Markov Model 69

    Figure 3.6: Function of the Mean Time to failure versus failure rate

    .. 70

    Figure 3.7: Markov Model with Continuous Parameter 70

    Figure 3.8: Diagram illustrating development of the mean transition time

    between states i and j . 71

    Figure 3.9: A simple maintenance model under deterioration failure. 73

    Figure 4.1: Block diagram showing the origin of Ikeja

    distribution zone . 83

  • xiii

    Figure 4.2: Line diagram showing 10 injection substations .. 84

    Figure 4.3: A section of the Abule-Egba distribution business unit 85

    Figure 4.4: A typical customer feeder in Ojokoro Substation 86

    Figure 4.5: Processed 2005 outage data for Abule-Egba business unit


    Figure 4.6: Processed 2006 outage data for Abule-Egba business unit


    Figure 4.7: Processed 2007 outage data for Abule-Egba business unit


    Figure 4.8: Processed 2008 outage data for Abule-Egba business unit


    Figure 4.9: Processed outage data for Ijaye Ojokoro feeders for 2005.92

    Figure 4.10: Processed outage data for Ijaye Ojokoro feeders for 2006.93

    Figure 4.11: Processed outage data for Ijaye Ojokoro feeders for 2007.


    Figure 4.12: Processed outage data for Ijaye Ojokoro feeders for 2008


    Figure 4.13a: Processed failure data for critical feeder for 2005....... 98

    Figure 4.13b: Processed failure data for critical feeder for 2006 99

    Figure 4.13c: Processed failure data for critical feeder for 2007 99

    Figure 4.13d: Processed failure for critical feeder for 2008 100

    Figure 5.1: Transformer maintenance model 109

    Figure 5.2: Perfect Maintenance Model 113

    Figure 5.3: Imperfect Maintenance Model 114

    Figure 5.4: Inspection Model 114

  • xiv

    Figure 5.5a-c: The relationship between inspection rate and MTTFF


    Figure 5.6a-c: The relationship between inspection rate and MTTFF

    When stage1 is represented by three subunits 119

    Figure 6.1: Stages in the asset Management lifecycle. 123

    Figure 6.2: Asset Management asset life cycle with about 90% Maintenance

    stage 124

    Figure 6.3: Transformer asset Management activities 125

    Figure 6.4: Transformer condition Monitoring and assessment techniques


    Figure 6.5: Classification of Maintenance activities 129

    Figure 6.6: Function of mean time to failure versus failure rate .. 132

    Figure 6.7 Markov Model with Continuous Parameter 132

    Figure 6.8: Markov Model for generating intensity Matrix 133

    Figure 6.9: Estimated transformer life-span at varying failure rates. 142

    Figure 6.10a-b: Sensitivity data fitted to 8th

    degree polynomial and its

    corresponding Norm residuals 142

    Figure 6.11: Plot of the result of the sensitivity analysis when other variables are

    held constant except maintenance rate () 145

    Figure 6.12a-b: Sensitivity data fitted to 3rd

    degree polynomial and its

    corresponding Norm residuals 145

  • xv


    Table 2.1 Changing maintenance techniques .. 33

    Table 4.1a: Statistical parameters from outage data set for 2004 96

    Table 4.1b: Statistical parameters from outage dataset for 2005 96

    Table 4.1c: Statistical parameters from outage dataset for 2006 96

    Table4.1d: Statistical parameters from outage dataset for 2007 97

    Table 4.1e: Statistical parameters from outage dataset for 2008 ..97

    Table 5.1: Number of failures for each cause of failure 105

    Table 5.2: List of the distribution of transformer failure by age .. 108

    Table 5.3: Transformer maintenance tasks 110

    Table 5.4: Rated limit for values of transformer oil for voltage class 111

    Table 5.5: List of model parameters and definitions 112



  • xvi


    The purpose of Maintenance is to extend equipment life time or at least the mean

    time to the next failure.

    Asset Maintenance, which is part of asset management, incurs expenditure but

    could result in very costly consequences if not performed or performed too little.

    It may not even be economical to perform it too frequently.

    The decision therefore, to eliminate or minimize the risk of equipment failure

    must not be based on trial and error as it was done in the past.

    In this thesis, an enhanced Reliability-Centered Maintenance (RCM)

    methodology that is based on a quantitative relationship between preventive

    maintenance (PM) performed at system component level and the overall system

    reliability was applied to identify the distribution components that are critical to

    system reliability.

    Maintenance model relating probability of failure to maintenance activity was

    developed for maintainable distribution components. The Markov maintenance

    Model developed was then used to predict the remaining life of transformer

    insulation for a selected distribution system. This Model incorporates various

    levels of insulation deterioration and minor maintenance state. If current state of

    insulation ageing is assumed from diagnostic testing and inspection, the Model is

    capable of computing the average time before insulation failure occurs.

    The results obtained from both Model simulation and the computer program of

    the mathematical formulation of the expected remaining life verified the

    mathematical analysis of the developed model in this thesis.

    The conclusion from this study shows that it is beneficial to base asset

    management decisions on a model that is verified with processed, analysed and

    tested outage data such as the model developed in this thesis.

  • 1



    1.1 Background Information

    Ability to use electrical energy when required is one of the fundamental

    presumptions of a modern society, and the introduction of complex and sensitive

    machines and systems into the network had increased the need for high reliability

    of supply [1]. Deregulation and competition are forcing improvements in efficiency

    and reductions in cost while customers are becoming more sensitive to electrical

    disturbances and are demanding higher levels of service reliability. Since a typical

    distribution system accounts for 40% of the cost to deliver power and 80% of

    customer reliability problems, distribution system design, operations and

    maintenance are critical for financial success and customer satisfaction [2].

    Moreover, failure statistics [3] reveal that the electrical distribution systems

    constitute the greatest risk to the uninterrupted supply of power. Traditionally

    however, distribution systems have received less attention than the generation and

    transmission parts of the overall electrical Power systems. This is emphasized by

    the clear difference in the number of publications within the various relevant fields


    The main reasons why distribution systems may not have been the centre of

    focus are that they are less capital-intensive and that their failures cause more

    localized effects compared to generation and transmission systems. However the

    focus on generation and transmission systems is moving toward distribution as the

    business focus changes from consumers to customers [4].

    Electrical power systems have undergone major changes during the last few years

    due to the introduction of the deregulated or liberalized market. (Sweden, for

    example, was one of the first countries to deregulate its power- supply market.

    This happened in January 1996) [5]. This has implied that the driving factors have

    moved from technical to economical. New players are now making their

    appearance in the field. This fundamental and global-level change in the running

  • 2

    of power utilities has brought about diversity effects, including new opportunities

    and new complications.

    These utilities are themselves active in the deregulated market and face various

    market challenges. For example, customers pay for energy delivered while

    authorities impose sanctions/regulations, they supervise and they compensate

    customers depending on the degree of fulfillment of contractual and other

    obligations as recommended [6,7].

    On the other hand, the owners expect the utilities to deliver at minimum

    cost. This means that electricity utilities must satisfy quantitative reliability

    requirements, while at the same time try to minimize their costs. One clear and

    predominant expenditure for a utility is the cost of maintaining system assets, for

    example through adopting preventive measures, collectively called preventive

    maintenance (PM). Preventive maintenance measures can impact on reliability by

    either, (a) improving the condition of an asset, or (b) prolonging the lifetime of an

    asset [8]. Reliability on the other hand, can be improved by either reducing the

    frequency or the duration of power supply interruptions.

    PM activities could impact on the frequency by preventing the actual cause of

    the failure. Consequently, in cost- effective expenditure, PM should be applied

    where the reliability benefits outweigh the cost of implementing the PM

    measures [9].

    Traditionally, preventive maintenance approaches usually consist of pre-defined

    activities carried out at regular intervals (scheduled maintenance). Such a

    maintenance policy may be quite inefficient; it may be costly (in the long run),

    and it may not even extend component lifetime as much as possible. In the past

    several years, therefore, many utilities replaced their maintenance routines based

    on rigid schedules by more flexible program using periodic or even continuous

    condition monitoring and data analysis [10]. Research findings have shown that

    maintenance impacts on the reliability performance of a component, that will

    eventually reflect on the entire system since power systems is made up of

    interconnected components[11]. Many programs had been used to validate this

  • 3

    fact, such as failure effects analysis, an evaluation of needs and priorities, and

    flow charts for decision making [12]. Some of these approaches have been

    collectively termed Reliability-Centred Maintenance (RCM) [13]. In a RCM

    approach, various alternative maintenance polices are compared and the most

    cost-effective is selected.

    RCM programs have been installed by many electric power utilities as a

    useful management tool [14]. However, the approach is still heuristic, and its

    application requires judgment and experience at every turn. Also, it can take a

    long time before enough data are collected for making such judgments. For this

    reason, several mathematical models have been proposed to aid maintenance

    scheduling [15].

    Many of these models [16] deal with replacement policies only and disregard

    the possibility of the cheaper but less effective maintenance activity. When

    maintenance is modeled, most often, fixed maintenance intervals are assumed.

    Only recently, was a mathematical model which incorporates the concept of

    maintenance when needed was developed. Detailed literature reviews on the

    various maintenance approaches and models are reported in references [17] and


    In this research work, a probabilistic model was developed for the failure

    and maintenance processes, and a Markov model for estimating the remaining

    life of an identified critical component of distribution network was also


    The model is based on a quantitative connection between reliability and

    maintenance, a link missing in the heuristic approaches. This model is capable

    of improving the decision process of a maintenance manager of network assets.

    RCM strategies that are capable of showing the benefits of performing cost

    effective PM on system networks on a selected system using reliability outage

    data were performed so as to identify the cirtical component for analysis. The

    model includes various levels of deterioration of the identified components as

    well as maintenance and inspection states. Assuming that the present state of

    component deterioration had been determined from diagnostic testing and

  • 4

    inspection, the model allows computation of the average time remaining before

    failure occurs using a computer program developed in Matlab. Reliability-

    centered Maintenance is a process used to determine the maintenance

    requirements of any physical asset in its operating context. This is based on

    equipment condition, equipment criticality and risk.

    RCM provides a tool for maintenance management (MM) by using the model

    to predict the remaining life of the identified component.


    This project work addresses the importance of maintenance on the reliability of

    electrical distribution systems. This focuses on preserving system function,

    identifying critical failure modes, prioritizing important components and selecting

    possible and effective maintenance activities, a cost effective preventive

    maintenance plan which defines reliability centred maintenance.

    1.2.1 Distribution Systems Constitutes The Greatest Risk.

    Electric power system is not 100% reliable. The ability to use electric energy when

    needed is the fundamental function of any modern utility company.

    The existence of sophisticated machines and production lines had increased the

    need for electricity supply that is highly reliable.

    Distribution aspect of electricity system had been identified as constituting the

    greatest risk to realizing uninterrupted power supply [19]. Studies show that a

    typical distribution system accounts for 40% of cost to deliver power and 80% of

    customer reliability problems [20]. This means that distribution systems are critical

    for financial success and customer satisfaction. And yet distribution systems have

    not received the desired attention. This was obvious from the difference in the

    number of publications.

    The main reasons advanced for the neglect of distribution systems include the


    They are less capital intensive

    Their failures cause more localized effects when compared with generation

    and transmission systems and

  • 5

    1.2.2 Introduction of Liberalised Market.

    The introduction of deregulated market has introduced major changes in electrical

    power systems. These changes had led to the movement of the driving factors from

    technical to economical. As a result, new investors are coming into the power

    sector. This global level change in the running of power sector has brought about

    new opportunities and new complications. In an increasingly competitive market

    environment where companies emphasize cost control, operation and maintenance

    (O&M), budgets are under constant pressure to economize. In order to ensure that

    changing utility environment does not adversely affect the reliability of customer

    power supply, several state regulatory authorities have started to specify minimum

    reliability standards to be maintained by the distribution companies [21].

    1.2.3 Cost-Effective Preventive Maintenance Expenditure

    Electric power utilities own and operate system generation, transmission and

    distribution of electricity. These utilities play active role in the deregulated market.

    The implication of this is that they also face market requirements. This means that

    customers will only pay for energy delivered. The Nigerian Electricity Regulatory

    Commission (NERC) which is the monitoring authority in Nigeria will imposes

    sanctions/regulations. Investors or Owners of utilities expect the managers to

    deliver electricity to customers at minimum cost. This means that utilities must

    satisfy reliability requirements at minimum cost. To achieve this, managers of these

    utilities must consider maintenance cost for system assets as an important

    expenditure area. Preventive maintenance measure is an activity undertaken

    regularly at pre selected intervals while the device is satisfactorily operating to

    reduce or eliminate the accumulated deterioration [22] while repair is the activity to

    bring the device to a non failed state after it has experienced a failure. When the

    cost incurred by a device failure is larger than the cost of preventive maintenance

    (this cost could be cost of downtime, repair expenses, revenue lost etc.), then it is

    worthwhile to carry out preventive maintenance.

    Preventive Maintenance measures can affect reliability in two ways:

    It helps in improving the condition of the asset and

  • 6

    It aids in prolonging the life time of an asset.

    Effects of high reliability are:

    It reduces the frequency of power outages by preventing the actual cause of


    It also reduces the duration of power supply interruptions.

    In cost effective expenditure, preventive maintenance applies where reliability

    benefits outweigh the cost of implementing the preventive maintenance measures.

    Traditional preventive maintenance is made up of pre defined activities carried

    out at regular intervals. This type of maintenance is costly, inefficient and may not

    even extend component lifetime. Many modern utilities have now replaced their

    routine maintenance that is based on rigid schedules with a more flexible program

    using periodic or even continuous condition monitoring (predictive maintenance).

    The predictive maintenance routines include group of programs such as Failure

    Modes and Effects Analysis, Evaluation of Needs and Priorities, and Flow Charts

    for Decision Making are some approaches that have been named reliability- centred

    maintenance (RCM) [23].

    In RCM approach, different maintenance policies can be compared and the most

    cost effective for sustaining equipment reliability selected. Reliability-Centred

    Maintenance program is not new. This program has been installed by some electric

    utilities as a useful management tool [24]. The problem with those in existence is

    that they cannot predict the effect of a given maintenance policy on reliability

    indicators (failure rate, outage time etc) and the approach adopted is still heuristic.

    This means that RCM in existence does not solve the fundamental problem of how

    the system reliability is impacted by component maintenance.The application is

    still based on experience and judgement at every turn. It takes a long time before

    enough data are collected for making such judgements. To solve one of the

    identified problems above, a probabilistic representation of deterioration process is

    modeled. A new mathematical formulation of the expected transition time from any

    deterioration state to the failure state (expected remaining life) has been presented.

    Processed outage data obtained from a selected distribution network for a

  • 7

    component was used as input on the Markov model developed to predict the

    remaining life. This predicted computation is executed using computer program

    developed in Matlab.

    Three stages will be used to describe deterioration process.

    Stage 1 represents an initial stage (D1).

    Stage 2 represents a minor deterioration stage (D2)

    Stage 3 represents a major stage of deterioration (D3).

    The last stage is followed in due time by equipment failure (F) which requires an

    extensive repair or replacement. Maintenance is carried out on asset to slow down

    deterioration. Inspections are performed so that decisions on asset management can

    be taken. To run this model however, it was assumed that repair after failures

    returns the device to the initial stage (as new condition). Figure 1.1 represents the

    conceptual diagram of the probabilistic model.

    Figure 1.1 conceptual diagram of the probabilistic model

  • 8


    The aim of this research study is to develop an appropriate method that will aid

    strategies for asset management in electric power distribution systems.

    These methods/strategies when developed should be cost effective, balancing the

    benefits in system reliability against the cost of maintenance methods. This will

    lead to the utilization of the reliability-centred maintenance (RCM) method. This

    method will be applied to specific parts in electrical power distribution systems.

    The main objectives are to

    a. Determine present maintenance policies in a selected distribution network.

    b. Develop a probabilistic based model for maintenance strategies.

    c. Predict the probable time of component failure, given that a certain stage in

    the ageing process has already been reached.

    d. Develop a quantitative relationship between preventive maintenance of

    system components and overall system reliability.

    e. Evaluate cost implications in the formulation of cost effective PM

    strategies and

    f. Conduct program evaluation, including general application to electrical

    power distribution systems.


    To fulfill the objectives of this work, the following methods will be adopted:

    The first phase of the work is the system reliability analysis. This involves the

    definition of the system and the identification of the critical components affecting

    system reliability.

    The second phase of the work is component reliability modeling. This entails

    detailed analyses of the components with the support of appropriate input data

  • 9

    collected with the use of questionnaire. This will define the quantitative

    relationship between reliability and preventive maintenance measures.

    The third and final phase are system reliability and cost/benefit analyses: This is

    carried out by putting the result of phase 2 into a system perspective, the effects of

    component maintenance on system reliability will then be evaluated and the impact

    on costs of different preventive maintenance strategies can now be identified.


    No doubt the country is in energy crisis, and the need to increase generation,

    manage and upgrade the existing power infrastructure becomes imperative. The

    costs of electric power outage to electric customers are enormous. Studies have

    shown [25] that the cost of electricity failures on the Nigerian manufacturing sector

    is quite high, as industries and firms incur huge costs on the provision of expensive

    back up to minimize the expected outage cost. The average costs of this back up

    are about three times the cost of publicly supplied power [25].

    The main function of power utility is to supply customers with electrical energy at

    high level of reliability at a reasonable cost. This intended function could be

    affected by the problem of power outage, which is one of the measures of reliability


    Power outage can; in principle be reduced in two ways:

    By reducing the frequency of interruptions, that is the number of failures, or

    By reducing the outage time, that is the duration of failure.

    Application of RCM technique will be used to address the first aspect above, which

    provides the focus for this study.


    Electricity is an aspect of the utility sector that is very essential to the smooth and

    meaningful development of a society. It supports the economy and promotes the

    well-being of individuals. Non-availability of this utility had led to a lot of

    challenges ranging from lack of foreign investment, high cost of living since most

  • 10

    manufacturers depend on private generators, high rate of unemployment and

    security and environmental hazards resulting from individuals generating their own

    electricity without regulations.

    A survey by Manufacturers Association of Nigeria (MAN) on power supply by the

    power Holding Company of Nigeria (PHCN) to industrial sectors in the first

    quarter of 2006 indicates that the average power outages increased from 13.3 hours

    daily in January to 14.5 hours in March 2006 [26].

    As at July 2009, Nigeria has total installed capacity of approximately 7060MW,

    however, the country is only able to generate between 800MW 4000MW from

    the seven major power stations and a numbers of IPP projects, because most of

    these facilities have been poorly maintained. Nigeria has plans to increase

    generation to 10,000MW by 2010. This means additional power plants, more

    transmission lines, as well as more distribution facilities.

    In recent times, subsequent governments of Nigeria had been working very hard to

    see the realization of steady power supply in the country. For example, the

    government of Chief Olusegun Obansanjo wanted to ensure an uninterrupted power

    supply by the end of 2001 in Nigeria. The president then, made it clear when he

    gave a mandate to the then National Electric Power Authority (NEPA) to ensure

    uninterrupted power to the nation by 31st December 2001. It was noted that NEPA

    actually raised electricity output from as low as 1,600 to 4,000MW and spent over

    one million dollars to meet this mandate [27].

    The present government also aware of this re-occurring power problem now

    declared during his campaign days that he will declare a state of Emergency on

    power sector when he assumed power. Yet as at February, 2010, Electricity

    reliability and availability are still a mirage.

    In this context, in as much as efforts are made towards efficient power generation,

    the subsequent transmission and distribution of the generated power should not be

    overlooked. Efficient utilization of the generated power cannot be achieved without

    a sound maintenance plan and monitoring of the transmission and distribution

    network system. Any organization that expects to run an efficient day to day

    operation and to manage and develop its services effectively must know what

  • 11

    assets it has, where they are, their condition, how they are performing, and how

    much it costs to provide the service [28]. Knowledge about the physical assets of

    the system is necessary to make strategic and maintenance/operation decisions.

    Thus, to make an intelligent decision vital to the smooth operations, growth and

    management of electricity distribution facilities, such decision must be based on a

    model that is verifiable and quantifiable and should not be decisions based on

    experience alone. This is the motivation for this project.


    i. A selective maintenance method based on reliability analysis is being


    ii. A Markov model for estimating the remaining life of a distribution

    transformer is implemented using Matlab program.

    iii. This will provide objectivity by converting the operators intuition into

    quantifiable values that will aid in decision making process for asset



    This research work identified the following two ways in which the reliability of

    electric supply to customers can be improved:

    i. Reducing the frequency of power outages or

    ii. Reducing the duration of power supply interruption.

    This research work covers the first part that uses reliability centred maintenance

    (RCM) to minimize the frequency of power outages by preventing the actual cause

    of failure. This is shown in figure 1.2. Maintenance (one of the main tools of asset

    management) in this context is considered as an activity of restoration where an

    unfailed device has its deterioration arrested, reduced or eliminated

    Its goal is to increase the duration of useful component life and postpone failures

    that would require expensive repairs. For a successful operation of this RCM plan,

    the degree of risk of each fault should be identified in order to define the optimum

    maintenance actions. The type of maintenance action to be taken for a particular

  • 12

    asset will depend on the risk index of that asset. Critical component will be

    identified from a selected network and the Markov model developed will be

    applied on the identified component to predict the remaining life so as to make

    intelligent decision on the asset.



    assessment of


    power System


    Figure 1.2 Project scope definitions


    The overall thesis structure can be broken down into individual chapters as follows:

    Chapter 1 provides an introduction, background studies, research methods and the

    main contributions that are unique to this work.

    Chapter 2 introduces and defines fundamental concepts for the analysis that


    Chapter 3 introduces basic evaluation methods and techniques for reliability

    modeling and analysis.

    Chapter 4 presents the computer program developed for reliability analysis of the

    electric power system.

    Chapter 5 introduces different maintenance procedures/strategies and provides

    introduction to reliability centred maintenance method (RCM) as applied to a

    distribution network.

  • 13

    Chapter 6 presents results from comprehensive study of the causes of failures in the

    identified critical components and then defines a model for estimating the

    remaining life of the identified distribution transformer.

    Chapter 7 concludes the work by summarizing the results obtained.

    Recommendations and issues for future work are identified and discussed.

  • 14



    2.1 Introduction

    Asset management (AM) is a concept used today for the planning and operation of

    the electrical power system. The aim of AM is to handle physical assets in an

    optimal way in order to fulfill an organizations goal while at the same time

    considering risk.

    One of the major risks that are involved in asset management is the probability of

    failure occurrence and its consequence. The goal is to ensure maximum asset value,

    maximum benefit or minimal life cycle cost.

    The only constraint to actualizing this goal is set on availability of revenues or

    power supply. There are different possible actions of handling these assets: They

    can either be acquired, maintained or replaced/redesigned.

    Maintenance management (MM) is therefore defined as a strategy to handle

    decisions for these assets and to make right decisions on

    what assets to apply actions to.

    what actions to apply

    how to apply the actions

    when to apply the actions

    The purpose of maintenance is to extend equipment life time or at least the mean

    time to the next failure whose repair may be costly. Further more, it is expected that

    effective maintenance policies can reduce the frequency of service interruptions

    and the many undesirable consequences of such interruptions. Maintenance clearly

    affects component and system reliability: if too little is done, this may result in an

    excessive number of costly failures and poor system performance and therefore

    reliability is reduced: When done too often, reliability may improve, but the cost of

    maintenance will sharply increase. In cost effective scheme, the two expenditures

    must be balanced.

  • 15

    Maintenance is just one of the tools for ensuring satisfactory component and

    system reliability. Others include increasing system capacity, reinforcing

    redundancy and employing more reliable components. At a time, however, when

    these approaches are heavily constrained, electric utilities are forced to get the most

    out of the system they already own through more effective operating policies,

    including improved maintenance programs. In fact, maintenance is becoming an

    important part of what is often called asset management.

    Electric utilities have always relied on maintenance programs to keep their

    equipment in good working conditions for as long as it is feasible. In the past,

    maintenance routines consisted mostly of pre-defined activities carried out at

    regular intervals. (Scheduled maintenance). However such a maintenance policy

    may be quite inefficient, it may be too costly (in the long run) and may not extend

    component life time as much as possible. In the last ten years, many utilities

    replaced their fixed interval maintenance schedules with more flexible programs

    based on an analysis of needs and priorities, or on a study of information obtained

    through periodic or continuous condition monitoring (predictive maintenance).[29]

    The predictive maintenance routines include a group of programs named

    Reliability-Centred Maintenance, [RCM]. In an RCM approach, various alternative

    maintenance policies are compared and the most cost-effective for sustaining

    equipment reliability selected. RCM programs have been installed by several

    electric utilities as a useful management tool.

    The implementation of RCM programs represented a significant step in the

    direction of getting the most out of the equipment installed. However, the

    approach and procedure is still heuristic and its application requires experience and

    judgment at every turn [30]. Besides, it can take a long time before enough data are

    collected for making such judgments. For this reason, several mathematical models

    have been proposed to aid maintenance scheduling [22, 23, 24 and 30].

    This chapter, gives a brief review of the most important approaches and models

    described in the literatures. Next, present maintenance policies are then examined.

    Subsequently, the use of mathematical models for maintenance strategies is

  • 16

    explored and desirable attributes of realistic probability-based models are listed. In

    closing, definitions of the most important concepts discussed in the work are given.


    A classification of the various maintenance approaches is presented in figure 2.1.

    Maintenance is shown here as part of the overall asset management effort.

    Maintenance policy is one of the operating policies and, in a given setting; it is

    selected to satisfy both financial constraints.


















    Figure 2.1 Overview of maintenance approachesMost of the discussion in the literature concerns replacements only, both after

    failures and during maintenance, and they disregard the possibility of the kind of

    maintenance where less improvement is achieved at smaller cost. The oldest

    replacement schemes are the age replacement and bulk replacement policies [31].

    In the first, a component is replaced at a certain age or when it fails, whichever

    comes first. In the second, all devices in a given class are replaced at predetermined

    intervals or when they fail. The last policy is easier to administer (especially if the

    ages of components are not known) and may be more economical than a policy

    based on individual replacement. Newer replacement schemes are often based on

  • 17

    probabilistic models [31] [32] and can be quite complex. In most electrical utility

    applications, however, maintenance resulting in limited improvement is an

    established practice and replacement models have only a secondary role.

    Maintenance programs range from the very simple to the quite sophisticated. The

    simplest plan is to adopt a rigid maintenance schedule where pre-defined activities

    are carried out at fixed time intervals. Whenever the component fails, it is repaired

    or replaced. Both repair and replacement are assumed to be much more costly than

    a single maintenance job. The maintenance intervals are selected on the basis of

    long-time experience (not necessarily an inferior alternative to mathematical

    models). To this day, this is the approach most frequently used.

    The RCM approach referred to in the introduction is heavily based on regular

    assessments of equipment condition and, therefore, does not apply rigid

    maintenance schedules. The term RCM identifies the role of focusing maintenance

    activities on reliability aspects. The RCM methodology provides a framework for

    developing optimally scheduled maintenance programs. The aim of RCM is to

    optimize the maintenance achievements (efforts, performance) in a systematic way.

    This method requires maintenance plans and leads to a systematic maintenance

    effort. Central to this approach is identifying the items that are significant for

    system function. The aim is to achieve cost effectiveness by controlling the

    maintenance performance, which implies a trade-off between corrective and

    preventive maintenance and the use of optimal methods.


    The RCM concept originated in the civil aircraft Industry in the 1960s with the

    creation of Boeing 747 series of aircraft (the Jumbo). One prerequisite for obtaining

    a license for this aircraft was having in place an approved plan for preventive

    maintenance (pm). However, this aircraft type was much larger and more complex

    than any previous aircraft type, thus PM was expected to be very expensive.

    Therefore it was necessary to develop a new PM strategy. United Airlines led the

    developments and a new strategy was created. This was primarily concerned

    with identifying maintenance tasks that would eliminate the cost of

    unnecessary maintenance without decreasing safety or operating performance.

  • 18

    The resulting method included an understanding of the time aspects in reliability

    (ageing) and identifying critical maintenance actions for system functions. The

    maintenance program was a success. The good outcome raised interest and the

    program spread. It was further improved, and in 1975 the US Department of

    commerce defined the concept as RCM and declared that all major military systems

    should apply RCM. The first full description was published in 1978 [33], and in the

    1980s the Electric Power Research Institute (EPRI) introduced RCM to the Nuclear

    power industry. Today RCM is under consideration by, or has already been

    implemented by many electrical power utilities for managing maintenance



    RCM provides a framework which enables users to respond to maintenance

    challenges quickly and simply. It does so because it never loses sight of the fact

    that maintenance is about physical assets. If these assets did not exist, the

    maintenance function itself would not exist. So RCM starts with a comprehensive,

    zero-based review of maintenance requirements of each asset in its operating


    All too often, these requirements are taken for granted. This results in the

    development of organization structures, the deployment of resources and the

    implementation of systems on the basis of incomplete or incorrect assumptions

    about the real needs of the assets. On the other hand, if these requirements are

    defined correctly in the light of modern thinking, it is possible to achieve quite

    remarkable step changes in maintenance effectiveness.

    The meaning of maintenance is explained. It goes on to define RCM and to

    describe the seven key steps involved in applying this process.

    Maintenance and RCM

    Considering the engineering view points, there are two elements to the

    management of any physical asset. It must be maintained and from time to time it

    may also need to be modified.

  • 19

    2.4.1 What Is Maintenance?

    Every one knows what maintenance is, or at least has his own customized

    definition of maintenance. If the question is asked, words like fix, restore, replace,

    recondition, patch, rebuild, and rejuvenate will be repeated. And to some extent,

    there is a place for these words or functions in defining maintenance. However, to

    key the definition of maintenance to these words or functions is to miss the mark in

    understanding maintenance especially if you wish to explore the philosophical

    nature of the subject.

    Maintenance is the act of maintaining. The basis for maintaining is, to keep,

    preserves, and protect. That is, to keep in an existing state or preserve from failure

    or decline. There is a lot of difference between the thoughts contained in this

    definition and the words and functions normally recalled by most people who are

    knowledgeable of the maintenance function, ie fix, restore, replace, recondition,


    Maintenance can therefore be defined, as ensuring that physical assets continue to

    do what their users want them to do.

    What the users want will depend on exactly where and how the asset is being used

    (the operating context). Maintenance procedures are an integrated part of the

    planning, construction and operation of a system. Moreover they are central and

    crucial to the effective use of available equipment. The aim of maintenance

    activities is to continuously meet performance, reliability and economic

    requirements, while also adhering to the constraints set by system and customer

    requirements. [34].

    The maintenance concept refers to all actions undertaken to keep or restore

    equipment to a desired state. The electrical power systems must abide by the

    regulations and norms for heavy current and maintenance, and in Nigeria, must

    follow the IEE regulations standard code. The IEE standard is the regulation

    governing the planning, building and maintenance of power distribution systems

    for 0.415 33kV. The first IEE standard regulation was created in the 1960s and

    the new handbooks have recently been developed to support more effective

    maintenance [35].

  • 20

    The cost of maintenance must be taken into consideration when handling system

    assets to minimize the lifetime costs of the system. However, some maintenance

    activities must be undertaken even if they are not profitable, such as earth plate

    metering inspections stipulated in the IEE regulations for power system. [36].

    There are two types of maintenance: Preventive Maintenance and Corrective


    Preventive Maintenance can be planned and scheduled, but corrective maintenance

    occurs unpredictably when failures are detected. This thesis focuses on preventive

    maintenance (PM).

    2.4.2 Maintenance Approaches

    From a basic point of view, there are two maintenance approaches. One approach is

    reactive and the other is proactive. In practice, there are many combinations of the

    basic approaches. The reactive system whose model is shown in figure 2.2

    responds to the following:

    a work request order

    Production staff identified needs.

    Failed system or its component.

    The effectiveness of this system will depend on response measures. The goals of

    this approach are to reduce response time to a minimum and to reduce equipment

    down time to an acceptable level.

    This is the approach used by most operations today. It may well incorporate what is

    termed as a preventive maintenance program and may use proactive technologies.

  • 21












    The proactive approach (figure 2.3) responds primarily to equipment assessment

    and predictive procedures. The overwhelming majority of corrective,

    preventative and modification work is generated internally in the maintenance

    function as a result of inspections and predictive procedures.

  • 22

    The goals of this method are continuous equipment performance to established

    specifications, maintenance of productive capacity, and continuous improvement.



















    Work order
















    Figure 2.3 Proactive Maintenance Model


    2.4.3 Changing Maintenance Trends.

    The International Council on Large Electric Systems (CIGRE) is one of the leading

    worldwide Organizations on Electric Power Systems with headquarters in France.

    This is a permanent non-governmental and non-profit international association that

    was founded in 1921. One of CIGRE core mission issues is related to the planning

  • 23

    and operation of power systems, as well as design, construction and maintenance of

    the plants. Technical work is being carried out within 15 study committees.

    One of these working groups set up a questionnaire in 1997 to obtain more

    information about trends in future power system planning, design, operation,

    maintenance, extension and refurbishment.

    A summary of this report can be found in reference [37], based on about 50

    responses obtained from utilities, manufacturers and consultants. Some of the

    results of particular importance to this context are pointed out in the following


    It is evident in the results that utilities have changed their organization in response

    to deregulation. The primary changes of note include the privatization of companies

    and splitting up of generation and distribution activities. The intense pressure to

    reduce operational and maintenance costs has already been felt. Maintenance,

    design, construction and some aspect of operation are increasingly being contracted

    out. The driving forces behind these changes are more aligned institutional business

    and economic factors than technical considerations.

    Another projected trend identified in the results is that manufacturers will become

    increasingly incorporated into the maintenance systems.

    Some of the figures presented in the report are as shown below:

    Almost 40% of the utilities undertake their maintenance activities at fixed

    time intervals and 30% on monitoring conditions. Many utilities falling into

    the first category are evolving towards condition or system- reliability based

    maintenance, or both.

    About half the utilities and all the manufacturers that responded have

    performed reliability studies to optimize their maintenance. These reliability

    studies resulted in introducing more flexibility and diversity into the

    maintenance intervals.

    In the past, utilities have laboured to achieve maximum reliability.

    However, according to the responses, about 90% thought that aiming for

    optimal and thereby more specific reliability in different parts of the

    system is the trend for the future.

  • 24

    Data concerning the times for repair and maintenance were stated to be

    available, but data on failure modes were claimed to be more difficult to


    This study provides a similar picture of the maintenance issue to that identified in

    the introduction of this thesis. It reveals a changing situation with increasingly

    complicated systems that are driven by economic rather than technical factors, and

    with the overall objective of achieving cost effective expenditures rather than

    maximum reliability.

    2.4.4 Changing Requirement for Maintenance Methods

    The change in the way maintenance is being managed has been identified. This

    change implies greater requirements on maintenance procedures. For example,

    maintenance decisions have been traditionally based on experiences and

    measurements which could be supported by diagnostic method. The increase in the

    expectations of maintenance has kept pace with the increasing knowledge about the

    dynamic characteristics of the power system. These higher expectations are due to

    the increasingly complex systems and higher demands on cost-effective use of

    resources. The increasing knowledge about the system has been gained primarily

    by an understanding of the relationships between failure frequency, reliability and

    maintenance, and also by methods and continuous measurements.

    2.4.5 Maintenance Specifications and Performance

    To explain maintenance specifications, maintenance definition will be considered

    in the context of keeping, preserving and protecting machine, equipment or plant.

    The challenge often faced in an attempt to perform these tasks is how to define the

    level to which the machine, equipment or plant is to be kept. One of the most

    common ways would be to say keep it like new. This sounds good, but it is more

    subjective than objective. To answer this issue of maintenance level, leads to

    maintenance specifications.

    Specification is a detailed precise presentation of that which is required. We must

    have a specification for the maintenance of equipment and plant. Specifications

    usually exist in the mind of the maintenance Engineer, even though they may be

    unable to recite it. This type of specification is defined in terms of and is dependent

  • 25

    upon time available, personnel training level, pressure to produce a current

    customer order now, money allocated or available, or management opinion.

    Obviously, a specification like this will not qualify as a true specification, nor will

    it qualify as a supporting component of the act of maintaining. The true

    maintenance specification may be a vendor specification, a design specification or

    an internally developed specification. The specification must be precise and

    objective in its requirement.

    The maintenance system and organization must be designed to support a concept

    based on acceptable standard. Specifications, detailed work plans and schedules

    may be constructed to provide the specification requirement at the maintenance

    level. In the maintaining context, specification is not a goal. It is a requirement that

    must be met. The maintenance system must be designed to meet this requirement.

    The specification must be accepted as the floor or minimum acceptable

    maintenance level. Variation that does occur should be above the specification

    level or floor. The specifications will probably be stated in terms of attributes and


    In reference to maintenance specifications, individual equipment specifications,

    process specification and plant performance specifications are also included.

    2.4.6 The Maintenance Function

    The maintenance department is responsible and accountable for maintenance. It is

    responsible for the way equipment runs and looks and for the costs to achieve the

    required level of performance. This is not to say that the operator has no

    responsibility for the use of equipment under his custody. The point is that

    responsibility and accountability must be assigned to a single function or person

    whether it is a mechanic or operator. To split responsibility between maintenance

    or any other department where overlapping responsibility occurs is to establish an

    operation where no one is accountable.

    The maintenance function is responsible for the frequency and level of

    maintenance. They are responsible for the costs to maintain, which requires

    development of detailed budgets and control of costs to these budgets.

  • 26

    Where the maintenance department or group is held responsible and accountable

    for maintenance, the relationship with other departments takes on new meaning.

    They must have credibility and trust as basis of interdepartmental relationships.

    This is an essential element for the successful operation of a maintenance

    management system.


    Most maintenance professionals are intimidated by the word reliability, because

    they associate reliability with RCM (Reliability-Centred Maintenance) and are

    unclear on what it actually means.

    Reliability is the ability of an item to perform a required function under a stated set

    of conditions, for a stated period of time [39]. However, many utilities focus on

    fixing equipment when it has already failed rather than ensuring reliability and

    avoiding failure.

    A common reason for this finding is the lack of time to investigate what is needed

    to ensure the reliability of equipment. Yet, a growing awareness among these

    reactive maintenance organizations is that the consequences of poor equipment

    performance include higher maintenance costs, increased equipment failure, asset

    availability problems and safety and environmental impacts. There is no simple

    solution to the complex problem of poor equipment performance. The traditional

    lean manufacturing or world class manufacturing is not the answer. These

    strategies do not address the true target, but if we focus on asset reliability, the

    result will follow.

    2.5.1 Reliability-Focus Utilities

    It is not possible to manage today power system operation with yesterday methods

    and remain in business tomorrow. Most chief executive of Companies that are

    doing well decide to focus on reliability because maintenance is the largest

    controllable cost in an organization [40] and, without sound asset reliability, losses

    multiply in many areas. A research carried out by over 50 key employees of the

  • 27

    worlds best maintenance organizations for a period of two years revealed the

    followings [41]:

    When the best practices they found were assimilated and implemented in a

    disciplined and structured environment, it was found to offer the biggest return with

    the longest lasting results.

    Corporations that truly understand reliability typically have the best performing

    plants. Some of the characteristics of reliability-focused organizations are

    Their goal is optimal asset health at an optimal cost.

    They focus on processes what people are doing to achieve results.

    They measure the effectiveness of each step in the process, in addition to

    the results.

    Their preventive maintenance programs focus mainly on monitoring and

    managing asset health.

    Their preventive maintenance programs are technically sound with each

    task linked to a specific failure mode, formal practices and tools are used to

    identify the work required to ensure reliability.

    2.5.2 System Functional Failure and Criticality Ranking.

    The objective of this task is to identify system functional degradation and failures

    and rank them as to priority. The functional degradation or failure of a system for

    each function should be identified, ranked by criticality and documented.

    Since each system functional failure may have a different impact on availability,

    safety and maintenance cost, it is necessary to rank and assign priorities to them.

    The ranking takes into account probability of occurrence and consequences of

    failure. Qualitative methods based on collective Engineering judgment and the

    analysis of operating experience can be used. Quantitative methods of simplified

    failure modes and effects analysis (SFMEA) or risk analysis also can be used.

    The ranking represents one of the most important tasks in RCM analysis. Too

    conservative ranking may lead to an excessive preventive maintenance program,

    and conversely, a lower ranking may result in excessive failures and potential

    safety impact. In both cases, a nonoptimized maintenance program results.

  • 28


    Reliability-centered Maintenance is a process used to determine the maintenance

    requirements of any physical asset in its operating context.

    2.6.1 RCM Method

    RCM provides a formal framework for handling the complexity of the maintenance

    issues but does not add anything new in a strictly technical sense. RCM principles

    and procedures can be expressed in different ways [42]; however, the concept and

    fundamental principles of RCM remain the same.

    The RCM method facilitates the

    preservation of system function,

    identification of failure modes,

    prioritizing of function needs, and

    selection of applicable and effective maintenance tasks.

    Several different formulations of the process of creating an RCM program and

    achieving an optimally-scheduled maintenance program were found in the

    literature. Three of these formulations had been addressed. The first two were

    derived from the original RCM definitions, and the third is an approach based

    on a set of questions rather than steps.

    1) Smith

    Smith defined a systematic process for RCM by implementing the following

    features that have been defined above:

    1. System selection and information collection.

    2. System boundary definition.

    3. System description and functional block diagrams.

    4. System functions and functional failures.

    5. Failure mode and effects analysis (FMEA).

    6. Logic decision tree analysis (LTA).

    7. Selection of maintenance tasks.

  • 29

    2) Nowlan

    Nowlan defines the process of developing an initial RCM program when the

    information required is lacking, as follows [43]:

    (1) Partitioning the equipment into object categories in order to identify those items

    that require intensive study,

    (2) Identifying significant items that have essential safety or economic

    consequences and hidden functions that require scheduled maintenance.

    (3) Evaluating the maintenance requirements for each significant item and

    hidden function in terms of the failure consequences and selecting only

    those tasks that will satisfy these requirements.

    (4) Identifying items for which no applicable or effective task can be found,

    then either recommending design changes if safety is involved, or assigning

    no scheduled maintenance tasks to these items until further information

    becomes available,

    (5) selecting conservative initial intervals for each of the included tasks

    grouping the tasks in maintenance packages for application,

    (6) Establishing an age-exploration program to provide the factual information

    necessary to revise initial decisions.

    The first step is primarily an activity for reducing the problem to a manageable

    size. The following three steps stated above are the essence of RCM analysis,

    constituting the decision questions as stated by Moubray in (3) below.

    3) Moubray

    To analyse the maintenance aspects of a system and its components, the first step is

    to identify the system items, and which of these ought to be analysed. Thereafter

    the RCM process can be formulated into seven questions for each of the selected

    items. [44]

    The seven general questions are:

    1. What are the functions and performances required?

    2. In what ways can each function fail?

    3. What causes each functional failure?

    4. What are the effects of each failure?

  • 30

    5. What are the consequences of each failure?

    6. How can each failure be prevented?

    7. How does one proceed if no preventive activity is possible?

    2.6.2 Failure Consequences

    A detailed analysis of an average industrial undertaking is likely to yield between

    three and ten thousand possible failure modes. Each of these failures affects the

    organization in some way, but in each case, the effects are different. They may

    affect operations. They may also affect product quality, customer service, safety or

    the environment. They will all take time and cost money to repair.

    It is these consequences which most strongly influence the extent to which we try

    to prevent each failure. In other words, if a failure has serious consequences, we are

    likely to go to great lengths to try to avoid it. On the other hand, if it has little or no

    effect, then we may decide to do no routine maintenance beyond basic cleaning and


    A great strength of RCM is that it recognizes that the consequences of failures are

    far more important than their technical characteristics. In fact, it recognizes that

    the only reason for doing any kind of proactive maintenance is not to avoid

    failures per se, but to avoid or at least to reduce the consequences of failure.

    The RCM process classifies these consequences into four groups, as follows:

    Hidden failure consequences: This has no direct impact, but they expose the

    organization to multiple failures with serious, often catastrophic,

    consequences. (Most of these failures are associated with protective devices

    which are not fail-safe.)

    Safety and environmental consequences: A failure has safety consequences

    if it could hurt or kill someone. It has environmental consequences if it

    could lead to a breach of any corporate, regional, national or international

    environmental standard.

  • 31

    Operational consequences: A failure has operational consequences if it

    affects production (output, product quality, customer service or operating

    costs in addition to the direct cost of repair).

    Non-operational consequences: Evident failures which fall into this

    category affect neither safety non production, so they involve only the

    direct cost of repair.

    2.6.3 Growing Expectation of Reliability-Centered Maintenance

    Since the 1930s, the expectation of maintenance can be traced through three

    generations. RCM is rapidly becoming a cornerstone of the third Generation, but

    this generation can only be viewed in perspective in the light of the first and second


    The first Generation

    The first generation covers the period up to World War II. In those days industry

    was not highly mechanized, so downtime did not matter much. This meant that the

    prevention of equipment failure was not a very high priority in the minds of most

    managers. At the same time, most equipment was simple and much of it was over-

    designed. This made it reliable and easy to repair. As a result, there was no need for

    systematic maintenance of any sort beyond simple cleaning, servicing and

    lubrication routines. The need for skills was also lower than it is today.

    The Second Generation

    Things changed dramatically during World War II. Wartime pressures increased

    the demand for goods of all kinds while the supply of industrial manpower dropped

    sharply. This led to increased mechanization. By the 1950s, machines of all types

    were more numerous and more complex. Industry was beginning to depend on


    As this dependence grew, downtime came into sharper focus. This led to the idea

    that equipment failures could and should be prevented, which led in turn to the

    concept of preventive maintenance. In the 1960s, this consisted mainly of

    equipment overhauls done at fixed intervals.

    The cost of maintenance also started to rise sharply relative to other operating

    costs. This led to the growth of maintenance planning and control systems. These

  • 32

    have helped greatly to bring maintenance under control, and are now an established

    part of the practice of maintenance.

    Finally, the amount of capital tied up in fixed assets together with a sharp increase

    in the cost of that capital led people to start seeking ways in which they could

    maximize the life of the assets.

    The Third Generation

    Since the mid-seventies, the process of change in industry has gathered even

    greater momentum. The changes can be classified under the headings of new

    expectations, new research and new techniques.

    Downtime has always affected the productive capability of physical assets by

    reducing output, increasing operating costs and interfering with customer service.

    By the 1960s and 1970s, this was already a major concern in the mining,

    manufacturing and transport sectors. In manufacturing, the effects of downtime are

    being aggravated by the worldwide move toward just-in-time systems, where

    reduced stocks of work-in-progress mean that quite small breakdowns are now

    much more likely to stop a whole plant. In recent times, the growth of

    mechanization and automation has meant that reliability and availability have now

    also become key issues in sectors as diverse as health care, data processing,

    telecommunications, power systems and building management.

    Greater automation also means that more and more failures affect our ability to

    sustain satisfactory quality standards. This applies as much to standards of service

    as it does to product quality.

    More and more failures have serious safety or environmental consequences, at a

    time when standards in these areas are rising rapidly. In some parts of the world,

    the point is approaching where organizations either conform to societys safety and

    environmental expectations, or they cease to operate. This adds an order of

    magnitude to our dependence on the integrity of our physical assets one which

    goes beyond cost and which becomes a simple matter of organizational survival.

    At the same time as our dependence on physical assets is growing, so too is their

    cost to operate and to own. To secure the maximum return on the investment

  • 33

    which they represent, they must be kept working efficiently for as long as we want

    them to.

    Finally, the cost of maintenance itself is still rising, in absolute terms and as a

    proportion of total expenditure. In some industries, it is now the second highest or

    even the highest element of operating costs [45]. As a result, in only thirty years it

    has moved from almost nowhere to the top of the league as a cost control priority.

    Table 2.1 Changing maintenance techniques

    First Generation:

    Fix it when


    Second Generation:

    Scheduled overhauls

    Systems for planning

    and conrolling


    Big, slow computers

    Third Generation:

    Condition monitoring

    Design for reliability and


    Hazard studies

    Small, fast computers

    Failure modes and effects analyses

    Expert systems

    Multiskilling and teamwork

    New research

    Apart from greater expectations, new research is changing many of our most basic

    beliefs about age and failure. In particular, it is apparent that there is less and less

    connection between the operating age of most assets and how likely they are to fail.

    However, Third Generation research has revealed that not one or two but six failure

    patterns actually occur in practice.

    New techniques

    There has been explosive growth in new maintenance concepts and techniques.

    Hundreds have been developed over the past fifteen years, and more are emerging

    weekly. [46].

  • 34


    The general power evaluation term reliability, such as availability, can be seen as

    a combination of three factors: Reliability of a piece of equipment or a part of the

    system, maintainability, which is the possibility to detect failures and to read and

    restore the components and the maintenance support or supportability i.e. spare

    parts, maintenance equipment and the ability of the maintenance staff. The

    availability concept and parameters of importance are illustrated in figure 2.4. All

    three areas are affected when underground cables replace overhead lines.

    80 % of the failures in distribution network are related to the electrical components,

    such as overhead lines, cable systems, secondary substations or medium voltage

    switchgear stations, [47] these components are made up of different parts of which

    all have a probability to fail. Cable system faults are not only faults on the cables

    but also on joints and terminations. In addition to the condition of individual

    components, network topology and environmental factors influence the ability of

    the system to perform a required function.

  • 35

    Availability Concept

















    FIGURE 2.4 Composition of availability and its controlling parameters.




    One approach to reliability engineering is to divide the process into four basic


    Past system behaviour

    Reliability calculation methods.

    Calculation of reliability indices and

    Prognosis of future system. [48]

    It is mainly the activities in step one, the collecting of data in order to create models

    of outages and failures, that differ between networks with an extensive amount of

    cable and traditional overhead line networks. The failure rates of different

    components, calculated in step one, are of the engineering process.

  • 36


    In most of the literature, the fundamental problem area considered is that of failure

    events in electric power systems. To make the analysis of this fundamental issue

    possible, abstract models were created using mathematical language instead of

    presenting the problem analogously. An abstract model can either be deterministic

    or probabilistic. In a deterministic model, reality can be approximated with a

    mathematical function. In a stochastic or random model, the unknown behaviour is

    included in the model. Probability theory is used to analysize